Keyboard: DKSANYKEY

       Name: DKSANYKEY                                               [Show more]
       Type: Subroutine
   Category: Keyboard
    Summary: Scan the keyboard to detect whether any key is being pressed
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


 Returns:

   X                    If a key is being pressed then X = $FF, otherwise X = 0

   A                    Contains the same as X


.DKSANYKEY

 LDX #0                 ; Set X = 0 as the value to return if no key is being
                        ; pressed

 BIT $C000              ; If bit 7 of the KBD soft switch is clear then there is
 BPL P%+6               ; no key press data to be read, so skip the next two
                        ; instructions to return a value of 0 in A and X

 DEX                    ; Otherwise bit 7 of the KBD soft switch is set, which
                        ; means there is a key bring pressed, so decrement X to
                        ; $FF so we can return this in A and X

 BIT $C010              ; Clear the keyboard strobe by reading the KBDSTRB soft
                        ; switch, which tells the system to drop any current key
                        ; press data and start waiting for the next key press

 TXA                    ; Copy the result into A

 RTS                    ; Return from the subroutine